Matrix2 serialisation has issues

i tried using the Matrix2 API and got some serialisation errors. I added a test case to reproduce the problem.

```
Caused by: com.esotericsoftware.kryo.KryoException: java.lang.NullPointerException
Serialization trace:
expressions (com.twitter.scalding.mathematics.Product)
$outer (com.twitter.scalding.mathematics.Product$$anonfun$toOuterSum$1)
fn (com.twitter.scalding.typed.MapFn)
```
Here is the full stack trace https://gist.github.com/stephanh/7332404.

Do you have any suggestions on how I can fix this?